With the development of modern mobile communication and microwave communication as well as demands for high-performance, low-noise and low-cost radio frequency (RF) components, traditional silicon devices can no longer meet new requirements on technical specifications, output power and linearity. Therefore, SiGe HBT devices have been proposed which play an important role in the applications of high-frequency power amplifiers. Compared with gallium arsenide (GaAs) devices, though SiGe HBT devices are at a disadvantage in frequency performance, they can well solve the issue of heat dissipation accompanying with power amplification, benefiting from their better thermal conductivities and good mechanical capacities of their substrates. Moreover, SiGe HBT devices also have better linearity and higher integration level. Further, SiGe HBT devices are well compatible with the conventional silicon process and still belong to the silicon-based technology and the complementary metal oxide semiconductor (CMOS) process, thus reducing manufacturing cost. For these reasons, the SiGe BiCMOS (bipolar complementary metal oxide semiconductor) process provides great convenience for the integration of power amplifiers and logic control circuits.
A zener diode is a diode whose operation relies on the voltage regulation characteristic of its PN junction's breakdown region. A zener diode is also called a voltage regulator diode, so as to be distinguished from those for unilateral conduction used in current regulation, detection and other applications. Thanks to the feature of a zener diode that the voltage across it remains substantially unchanged after it is broken down, when a zener diode is connected in a circuit, the voltage across the load can remain substantially unchanged even when the voltages at other nodes of the circuit are changed due to the power source voltage fluctuations or other reasons. When a zener diode is broken down in the reverse direction, though the current flows therein will undergo a great change, the voltage across the zener diode will change slightly. Because of such electrical characteristic, a zener diode plays a role of voltage regulation in a circuit and is mainly used as a voltage regulator or voltage reference element. Several zener diodes connected in series can produce a higher constant voltage to meet demand in high voltage applications.